Vitamin D is a fat-soluble vitamin that was isolated in 1930 and named calciferol. Since then more metabolites have been found, and the two major forms of this vitamin are now known to be vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D is actually a hormone precursor, which can be manufactured by the body. Therefore, in a classical sense, it is not actually an essential nutrient. However, since the disease rickets is related to vitamin D deficiency, it has been traditionally classified as a vitamin.

Vitamin D is known as the â€œsunshineâ€ vitamin. It is formed in the body by the action of the sunâ€™s ultraviolet rays on the skin, converting the biological precursor 7-dehydroergosterol (found in animals and humans) into vitamin D3. Vitamin D3 is converted in the liver to 25-hydroxycholecalciferol (25-HCC) which is five times more active than vitamin D3. 25-HCC is then converted in the kidneys to 1,25-dihydroxycholecalciferol (1,25-HCC) which is 10 times more potent than vitamin D3. The active 1,25-HCC form of vitamin D is also called calcitrol. Since calcitrol is produced in the kidney and functions elsewhere in the body, it is considered a hormone, with the intestines and bone as its target.

A study published in the Journal of Clinical Endocrinology and Metabolism sought to determine whether vitamins D2 and D3 are biologically equivalent. The single-blind, randomized study included 33 healthy adults who were supplemented with 50,000 IU of either vitamin D2 or D3 per week for a total of 12 weeks.

The results revealed that after 12 weeks of supplementation, serum vitamin D levels increased significantly more in the D3 group than in the D2 group. The results were vitamin D3Â was 56 to 87 percent more potent than D2 in raising serum vitamin D levels. It was also revealed that vitamin D3 produced a 2- to 3-fold increase in the storage of serum vitamin D, compared with vitamin D2. These findings suggest that vitamin D3 should be the preferred treatment for vitamin D deficiency due to its greater potency and lower cost.1